Philosophy is all about questioning reality. Epistemology is about questioning our knowledge of reality and metaphysics is about reality itself. Philosophical scepticism can lead people to make bizarre hypotheses, such as the idea that we might be brains in vats, or victims of Descartes’ evil demon, or whatever. So my question is: where does philosophical scepticism end and downright mental abnormality begin?

Solipsism, external world scepticism and the like are commonly considered by philosophers, and they are not considered delusional. And yet, ‘solipsism syndrome’ and ‘the Truman Show delusion’ are recognised as mental illnesses. The content of these beliefs seems similar, so why are solipsism and external world scepticism considered rational whereas solipsism syndrome and the Truman Show delusion considered mental illnesses?

The best answer I can think of would be that philosophical scepticism is based on rational questioning and that it is not firmly held, like a delusion would be. Whereas, the Truman Show delusion is fixed and is based on irrational inference from events to the belief that reality is not what it seems/reality is based around you/whatever. But this is not totally satisfying for me. How can we so clearly draw the line between “rational questioning” and “irrational inference”?

This question has some personal relevance to me because I am often led to think in unusual ways about things, and I am not sure whether I am being epistemological or whether I am downright mentally abnormal. For example, if I watch a film I have already seen, and something seems different from how I remember it, I can sometimes be led to wonder whether this could be like a sign from God/whatever that the reality I experience is not ‘true reality’, and God/whoever is 'up there' is trying to show me this, by showing me the internal contradictions inherent in the reality I experience. And later, I realise that a much more rational explanation would simply be that my memory was poor with regards to what happened in the film, and then it bothers me that I would even think along the ‘sign from God’ lines, and I am left genuinely frightened for my mental health. I have actually explained the way I sometimes think to two ‘professionals’ (a physician and a psychologist) who I was seeing for largely unrelated reasons, and they have both assured me that I am not crazy and that I don’t have schizophrenia or any serious mental illness.

Aside from the assurances of the physician and the psychologist, two things make me slightly more confident about my own sanity: 1. When I think in the way I described above, it is not as if my thoughts are fixed, deeply-held delusions which affect the way I live my everyday life, they are just momentary ideas which I later go on to self-analyse myself about; and 2. It is considered ‘normal’ to believe in ghosts and horoscopes and New Age bullshit, and how things like a baby surviving a tornado or the Pope surviving an assassination attempt are signs of divine intervention, which I think are at least as irrational as the way I sometimes think.

Still though, I have no idea where rational epistemological scepticism ends and psychiatric delusion begins. Does anybody have any better suggestions with regards to how to demarcate the two? And has anybody else been led to think in the way I described above, and been worried about whether they were being epistemological or whether they had something wrong with them?

A man sooner or later discovers that he is the master-gardener of his soul, the director of his life.

I think that skeptical philosophy implies doubt regarding what is seen, while a belief that one is in the Matrix implies a positive belief in what is not seen, and solipsism involves a positive belief that there is nothing but oneself. Skepticism is the exact opposite of positive belief. If one should be skeptical of what one has prima facie sensory evidence of, one should be triply skeptical of theories that one cannot have sensory evidence of.

I, Lex Llama, super genius, will one day rule this planet! And then you'll rue the day you messed with me, you damned dirty apes!

Read the conclusion of Book I of Hume's Treatise. That is the most famous discussion of your question by far. His philosophical skepticism is so great he is almost shaking. But then everyday life intervenes and all his doubts are set aside. Very famous section.

...I am first affrighted and confounded with that forelorn solitude, in which I am plac'd in my philosophy, and fancy myself some strange uncouth monster, who not being able to mingle and unite in society, has been expell'd all human commerce, and left utterly abandon'd and disconsolate. Fain wou'd I run into the crowd for shelter and warmth; but cannot prevail with myself to mix with such deformity. I call upon others to join me, in order to make a company apart; but no one will hearken to me. Every one keeps at a distance, and dreads that storm, which beats upon me from every side. I have expos'd myself to the enmity of all metaphysicians, logicians, mathematicians, and even theologians; and can I wonder at the insults I must suffer? I have declar'd my disapprobation of their systems; and can I be surpriz'd, if they shou'd express a hatred of mine and of my person? When I look abroad, I foresee on every side, dispute, contradiction, anger, calumny and detraction. When I turn my eye inward, I find nothing but doubt and ignorance. All the world conspires to oppose and contradict me; tho' such is my weakness, that I feel all my opinions loosen and fall of themselves, when unsupported by the approbation of others. Every step I take is with hesitation, and every new reflection makes me dread an error and absurdity in my reasoning....Most fortunately it happens, that since reason is incapable of dispelling these clouds, nature herself suffices to that purpose, and cures me of this philosophical melancholy and delirium, either by relaxing this bent of mind, or by some avocation, and lively impression of my senses, which obliterate all these chimeras. I dine, I play a game of backgammon, I converse, and am merry with my friends; and when after three or four hours' amusement, I wou'd return to these speculations, they appear so cold, and strain'd, and ridiculous, that I cannot find in my heart to enter into them any farther.....

I think (hmm, if I can cease thinking will that be me?) this insanity/wisdom duality is not as clear to draw a line. If Nietzsche is considered a mad thinker, well, initially irrational author and later years mentally ill, some put a line when he shed tear over a whipped horse. But, rationality is not without dispute. Dispute is not too bad. Heraclitus of Ephesus (do I put the names correctly) said something on war as father of all.

Or clearly, I live in the irrationale camp long enough to Think rationality is but flux.

Worry not, we shall pass into oblivion, ultimately. Crazy or not, be happy.